Simultaneous multilayer coating of aqueous gelatin/silver halide emulsions ("photographic emulsions") has been used extensively in the manufacture of photographic films. Photographic emulsions contain aqueous gelatin solutions containing dispersed silver halide grains. In color photographic emulsions, there are present color couplers which are spectrally matched to the sensitization of the silver halide grains. These color couplers are, in turn, contained in dispersed droplets of a water insoluble oil. The individual color coupler molecules have attached oleophilic "ballasting groups", such as tertiary amyl groups, which ensure that the coupler molecule remains dissolved in the oil droplet rather than dissolving into the aqueous phase from which it can undergo interlayer diffusion.
It is essential that the color couplers remain confined within their assigned layers in close association with their correspondingly sensitized silver halide grains. Were the coupler to migrate into a different color layer and react with the wrong silver halide grain, false color renderings would occur (commonly known as "crosstalk").
Simultaneous multilayer coating has the primary advantage of reducing the number of coating steps needed to prepare multi-layered articles. The process for simultaneously applying aqueous gelatin emulsions to form a multilayer film generally involves extruding gelatin emulsions at a temperature above their gel point and then simultaneously coating the extruded gelatin solutions onto a moving web using a coating apparatus (e.g., a slide-hopper). Upon contact with the web, the gelatin-based layers are rapidly cooled below their gel temperature, thereby gelling the individual layers (wherein a rapid qualitative change from liquid to solid properties occurs) and minimizing interlayer mixing, and drying related defects, especially mottle. Subsequently, the coated gelled film is dried to remove excess water. Until now, there has been no disclosure of simultaneously applying organic solvent-based coatings, which can be cooled to organogels, to suitable substrates.
U.S. Pat. No. 4,966,792 describes stacked aqueous gel-forming solutions (e.g., acrylamides) of varying concentration gradients for use in electrophoresis. There is no disclosure of using non-aqueous-based gels.
U.S. Pat. No. 4,525,392 discloses a method for simultaneously applying multiple layers of gelatin solutions to a web. A slide-hopper type coating apparatus is used to coat the solutions. Interlayer mixing is controlled by adjusting the relative flow viscosities of the aqueous gelatin layers flowing on the slide surface.
U.S. Pat. No. 4,384,015 and U.S. Statutory Invention Registration H1003 disclose processes for the simultaneous coating of multiple aqueous gelatin-based layers for photographic applications.
U.S. Pat. No. 3,920,862 discloses multilayer coating of aqueous gelatin solutions incorporating a stripe of recording material.
U.S. Pat. No. 4,791,004 discloses a method for forming multi-layered coated articles by increasing the viscosity of a coated solution followed by a lamination step.
U.S. Pat. No. 4,684,551 discloses an apparatus useful for coating thixotropic polyvinyl fluoride as a plastisol in a latent solvent (i.e., a liquid dispersing agent that becomes a true solvent upon heating). No mention of multiple coatings is made.
U.S. Pat. Nos. 2,647,296 and 2,647,488 disclose a method for coating textile fabric with a polymeric plastisol composition.
U.S. Pat. Nos. 2,419,008, 2,419,010, 2,510,783, 2,599,300, 2,953,818, and 3,139,470 disclose processes for the manufacture of films from orientable polyvinyl fluoride. Those processes involve extrusion of polyvinylidene fluoride dissolved in a solvent. A solvent is mixed with polyvinylidene fluoride and heated until the polyvinyl fluoride particles coalesce. The uniform mixture is extruded and upon rapid cooling forms a self-supporting film which can be further dried.
U.S. Pat. No. 4,281,060 discloses the use of polyisocyanate hardeners to improve multilayer coatability of silver halide-containing photothermographic layers having poly(vinyl butyral) binders.
European Patent Application No. 388,818 discloses a dual slot extrusion coating die for use with non-aqueous coating compositions. It is limited to the application of two layers to a continuously moving web.
What would be desirable in the industry is a process for the simultaneous application of multiple layers of thermoreversible organogels (as defined later herein) to suitable substrates with minimal intermixing of the polymeric layers or critical ingredients (either polymeric or supramolecular) dispersed or dissolved therein.